Steel manufacturers who participated in the New RC project made trial manufacture of re-bars conforming to the target performance by adopting two methods: the first one was by component adjustment and hot rolling (as roll) including on-line heat treatment in the rolling process, and another one was off-line heat treatment after completion of rolling. As stated previously, the New RC project adopted four grades of high strength steel as its aim of
development, all of which are required high degree of ductility in addition to high strength. In principle, three methods are conceivable to make high strength re-bars: addition of strengthening chemical elements, cold work, and heat treatment. However, a careful process design is necessary for each of these methods corresponding to employed equipment.
Adopted methods of manufacture for each of four grades are summarized below.
USD685A and USD685B: Since required yield plateau strain is large, and also required yield ratio is low, cold work is not suitable as it leads to un- clear yield point and reduced ultimate strain. Re-bars of this grade can be manufactured either by addition of strengthening chemical elements or by heat treatment. Addition of one or several kinds of strengthening chemical elements to molten steel results in higher strength due to atomic size effect (solid melt- ing or replacement) or crystallization effect. Higher yield stress and strain can be achieved by finer crystalline particles. By adding Al, Ti, or Nb at the steel manufacture, and by heating and hot roll, fine crystalline particles of austenite can be obtained. On the other hand, heat treatment of quenching and tem- pering can be employed to the ordinary medium carbon steel with addition of chemical elements effective for quenching. In either of these methods, amount of impure elements that affect mechanical properties must be carefully con- trolled.
USD980: This steel is high strength but required yield ratio is relatively high. Hence commercially available P C steel manufacturing technology of JIS G 3109 "PC bars" can be applied. First deformed bars are manufactured by adding chemical elements effective for heat treatment or cold work. Then heat treatment of quenching and tempering is conducted, or cold work by 10 percent stretching and subsequent brewing are applied, in order to secure high strength and ductility.
USD785: This steel is for small diameter bars to be used as lateral reinforce- ment. It can be manufactured by addition of strengthening chemical elements and by on-line heat treatment at the time of hot rolling consisting of air-cooling to quench during hot roll followed by tempering automatically by remaining heat.
USD1275: This steel is also for small diameter bars to be used as lateral reinforcement. It can be manufactured by methods specified in the current JIS G 3536 "PC steel wires and PC strands" or JIS G 3109 "PC steel bars".
Some products were already available commercially at the time of New RC project.
i Method of manufacture
Add strengthening elements
Size
D13 D22 D32 D41
Table 3.4. Trial manufactured USD685B.
Chemical component (% by weight) C
0.33 0.32 0.32 0.32
Si
0.41 0.41 0.99 0.99
Mn
0.75 0.70 1.58 1.55
P
0.007 0.010 0.006 0.009
S
0.004 0.001 0.002 0.004
Mechanical properties Yield
Point (MPa)
726 696 710 702
Tensile JYield strength ratio
(MPa)
111 1 0.80 0.79
0.79 0.78
Elon- gation
(*)
19 14 18 17
(MPa) 745 734 731 732
Bending
good good good good ]
*stress at e = 1.4%.
(a) (b)
(c) (A)
Fig. 3.29. (a) Tensile test of re-bar. (b) An example of stress-strain curve, (c) Microscopic structure, (d) Bending test.
A part of results of trial manufacture is introduced herein. Table 3.4 is the chemical components and mechanical properties of USD685B re-bars of four different sizes. It will be seen that all bars conform to requirements in Table 3.1.
Figures 3.29(a) and (b) show tensile test of re-bar and an example of stress- strain curve. Figure 3.29(c) shows microscope structure, and Fig. 3.29(d) shows results of 90 degree bending test. These are examples of trial manufacture
(b) Alternate reversal of loading
Fig. 3.30. Stress-strain curves under reversal of loading.
illustrating the possibility to make products of USD685B conforming to the proposed Standard. Figure 3.30 shows two examples of stress-strain curves under reversal of loading, (a) into one-way increase into tension, and (b) in alternative increase into both tension and compression. It will be seen that Bauschinger effect and strain hardening of the new steel are similar to those of currently used steel re-bars.
High-stress fatigue test, carried out assuming cyclic earthquake or wind loading, showed that for USD685, stress amplitude directly affected the fatigue strength. For stress amplitude of 0.98 and 0.93 times the specified yield strength, the number of cycle to failure changed from 6000 to 10 000. USD980 has higher yield ratio than USD685, and hence test stress was high. It appeared that the number of cycle to failure was affected more by the shape of surface deformation than the stress amplitude.
In case of design of structures susceptible to fatigue condition, it would be necessary to carry out fatigue test assuming the actual design condition to which high strength re-bars are exposed, considering the possibility of the influence of shape of surface deformation.